Solder bump structure and method for forming a solder bump

ABSTRACT

A solder bump structure includes a contact pad, an intermediate layer located over the contact pad, a solder bump located over the intermediate layer, and at least one metal projection extending upwardly from a surface of the intermediate layer and embedded within the solder bump. Any crack in the solder bump will tend to propagate horizontally through the bump material, and in this case, the metal projections act as obstacles to crack propagation. These obstacles have the effect of increasing the crack resistance, and further lengthen the propagation path of any crack as it travels through the solder bump material, thus decreasing the likelihood device failure.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to integrated circuit(IC) chips and devices, and more particularly, the present inventionrelates to solder bump structures of IC chips and devices, and tomethods of forming solder bump structures.

[0003] 2. Description of the Related Art

[0004] As integrated circuits (IC's) advance toward higher speeds andlarger pin counts, first-level interconnection techniques employing wirebonding technologies have approached or even reached their limits. Newimproved technologies for achieving fine-pitch wire bonding structurescannot keep pace with the demand resulting from increased IC chipprocessing speeds and higher IC chip pin counts. As such, the currenttrend is to replace wire bonding structures with other packagestructures, such as a flip chip packages and a wafer level packages(WLP).

[0005] Flip chip packages and WLP structures are partially characterizedby the provision of solder bumps which connect to interconnectionterminals of the IC chip. (Herein, unless otherwise specified, the termsolder “bumps” is intended to encompass solder “balls” as well.) Devicereliability is thus largely dependent on the structure and material ofeach solder bump and its effectiveness as an electrical interconnect.

[0006] A conventional solder bump structure will be described withreference to FIGS. 1 and 2, where like elements are designated by thesame reference numbers. FIG. 1 shows the state of a flip chip packageprior to mounting on a printed circuit board (PCB) substrate, and FIG. 2shows the flip chip package mounted on the PCB substrate.

[0007] In FIGS. 1 and 2, an integrated circuit (IC) chip 1 is equippedwith a chip pad 2, which is typically formed of aluminum. An opening isdefined in one or more passivation layers 3 and 4 which expose a surfaceof the chip pad 2. Interposed between a solder bump 5 and the chip pad 2are one or more under bump metallurgy (UBM) layers 6 and 7.

[0008] The UBM layers 6 and 7 functions to reliably secure the bump 5 tothe chip pad 2, and to prevent moisture absorption into chip pad 2 andIC chip 1. Typically, the first UBM layer 6 functions as an adhesionlayer and is deposited by sputtering of Cr, Ti, or TiW. Also typically,the second UBM layer 7 functions as a wetting layer and is deposited bysputtering of Cu, Ni, NiV. Optionally, a third oxidation layer of Au maybe deposited as well.

[0009] Referring to FIG. 2, the solder bump 5 is mounted to a PCB pad 8of a PCB substrate 9.

[0010] Mechanical stresses on the solder bump are a source of structuralwhich can substantially impair device reliability. That is, when thechip heats up during use, both the chip and the PCB expand in size.Conversely, when the chip cools during an idle state, both the chip andthe PCB substrate contract in size. The chip and the PCB substrate havemismatched coefficients of thermal expansion, and therefore expand andcontract at different rates, thus placing mechanical stress on theintervening solder bump. FIG. 3 illustrates a number of examples inwhich stresses have caused fissures to be formed in the solder bumps. Inthis figure, reference number 2 denotes a chip pad, reference number 5denotes the solder bump, reference number 8 denotes the PCB pad, andreference number 10 denotes a crack or fissure. The larger the crack,the more the interconnection becomes impaired, and device failures canoccur when cracks propagate completely through the solder bumpstructure.

SUMMARY OF THE INVENTION

[0011] According to a first aspect of the present invention, a methodfor manufacturing a solder bump is provided, which includes forming atleast one metal protrusion extending upwardly over a contact pad, andembedding the at least one metal protrusion in a solder material.

[0012] According to another aspect of the present invention, a methodfor manufacturing a solder bump is provided, which includes depositingan intermediate layer over a contact pad, forming a photoresist over asurface of the intermediate layer, patterning the photoresist to defineat least one opening which partially exposes the surface of theintermediate layer, filling the at least one opening of the photoresistwith a metal, removing the photoresist to expose the metal, wherein themetal forms at least one metal protrusion extending upwardly from thesurface of the intermediate layer, and embedding the at least one metalprotrusion in a solder material formed over the intermediate layer.

[0013] According to yet another aspect of the present invention, amethod for manufacturing a solder bump is provided, with includesdepositing an intermediate layer over a contact pad, forming aphotoresist over a surface of the intermediate layer, patterning thephotoresist to define at least one opening which partially exposes thesurface of the intermediate layer, filling the at least one opening ofthe photoresist with a metal to a first depth, filling the at least oneopening of the photoresist with a solder material to a second depth suchthat the solder material is stacked on the metal within each of the atleast one opening, removing the photoresist to expose the metal and thefirst solder material, wherein the metal and first solder material format least one protrusion extending upwardly from the surface of theintermediate layer, and reflowing the solder material to form a solderbump having the metal embedded therein.

[0014] According to still another aspect of the present invention, amethod for manufacturing a solder bump is provided, which includesdepositing an intermediate layer over a contact pad, forming aphotoresist over a surface of the intermediate layer, patterning thephotoresist to expose a solder bump region over the surface of theintermediate layer, growing at least one metal dendrite having aplurality of branches which project upwardly in the solder bump regionfrom the surface of the intermediate layer, filling the solder bumpregion with a solder material so as to embed the at least one metaldendrite within the solder material, and removing the photoresist.

[0015] According to another aspect of the present invention, a solderbump structure is provided, which includes a contact pad, anintermediate layer located over the contact pad, a solder bump locatedover the intermediate layer, and at least one metal projection extendingupwardly from a surface of the intermediate layer and embedded withinthe solder bump.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The aspect and advantages of the present invention will becomereadily apparent from the detailed description that follows, withreference to the accompanying drawings, in which:

[0017]FIG. 1 illustrates a conventional flip-chip structure prior tomounting on a printed circuit board substrate;

[0018]FIG. 2 illustrates a conventional flip-chip structure aftermounting on a printed circuit board substrate;

[0019]FIG. 3 contains photographs of conventional structures in whichfissures have developed within the solder bumps thereof;

[0020]FIG. 4(a) is a cross-sectional view of a solder bump structureaccording to an embodiment of the present invention;

[0021]FIG. 4(b) a cross-sectional view along the line I-I′ of FIG. 4a;

[0022] FIGS. 5(a) through 5(i) are cross-sectional views for use indescribing a process for manufacturing a solder bump structure accordingto a preferred embodiment of the present invention;

[0023] FIGS. 6(a) through 6(g) are cross-sectional views for use indescribing a process for manufacturing a solder bump structure accordingto another embodiment of the present invention;

[0024]FIG. 7(a) is a cross-sectional view of a solder bump structureaccording to another embodiment of the present invention;

[0025]FIG. 7(b) is a cross-sectional view along the line II-II′ of FIG.7a; and

[0026] FIGS. 8(a) and 8(b) are cross-sectional views of solder bumpstructure having dendrite configuration embedded therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention is at least partially characterized by theinclusion of one or more metal projections within the solder bumpmaterial to form an obstacle which impedes the propagation of a crackwithin the solder bump material. While the metal projections can takeany number of forms, the invention will be described below withreference to several preferred embodiments.

[0028]FIG. 4(a) is a cross-sectional view of a solder bump structureaccording to an embodiment of the present invention, and FIG. 4(b) across-sectional view along the line I-I′ of FIG. 4(a). The solder bumpstructure includes a contact pad 402 of an electronic device such as anIC chip 401. Preferably, the IC chip 401 is contained a flip chippackage or a wafer level package. An opening is defined in one or morepassivation layers 403 and 404 which expose a surface of the chip pad402. Interposed between a solder bump 405 and the chip pad 402 are oneor more intermediate layers 406 and 407. The intermediate layers 406 and407 may be under bump metallurgy (UBM) layers. For example, the layer406 may be a UBM adhesion layer made of Cr, Ni or TiW, and the layer 407may be a wetting layer made of Cu, Ni or NiV. Also, an additionaloxidation layer of Au may be provided.

[0029] The solder bump 405 is located over the intermediate layer 407.An example of the solder bump dimensions is 100×100 μm, and examples ofa material constituting the solder bump include Sn, Pb, Ni, Au, Ag, Cu,Bi and alloys thereof. In addition, at least one metal projection 411extends upwardly from a surface of the intermediate layer 407 and isembedded within the solder bump 405. In this embodiment, as shown inFIGS. 4(a) and 4(b), a plurality of metal projections 411 extendupwardly from the surface of the intermediate layer 407 and are embeddedwithin the solder bump 405. Preferably, because of reflow of the soldermaterial during fabrication, a melting point of a material of the solderbump 405 is less than a melting point of the metal projections 411. Anexample of the width of each projection 411 is about 5 to 70 μm, andexamples of the material constituting the metal projections 411 includeNi, Cu, Pd, Pt and alloys thereof.

[0030] As best shown in FIG. 4(b), a cross-section of the plurality ofprojections 411 defines a regular mesh pattern in a plane parallel tothe contact pad 402. Generally, any crack in the solder bump will tendto propagate horizontally through the bump material, and accordingly,the regular mesh pattern of metal projections 411 act as obstacles tocrack propagation. These obstacles have the effect of increasing thecrack resistance, and further lengthen the propagation path of any crackas it travels through the solder bump material, thus decreasing thelikelihood device failure.

[0031] Many other patterns of metal projections 411, both regular andirregular, may be adopted, such as off-set parallel rows of projectionsor concentric sets of patterns. In addition, the individual projections411 may have cross-sections which differ from the generally squarecross-sections of FIG. 4(b), such as elliptical cross-sections,polygonal cross-sections, and combinations thereof. Moreover, a singlecontiguous projection may be provided, for example in the shape of aspiral or zig-zag pattern. Finally, the invention is not limited to thecolumnar projections 411 having vertical sidewalls as illustrated inFIG. 4(a). For example, projections with oblique or notched sidewallsmay be formed instead. Also, projections having non-regular geometricshapes may be provided, such the dendrite structures described in alater embodiment.

[0032] A method for manufacturing a solder bump structure according toan embodiment of the invention will now be described with reference toFIGS. 5(a) through 5(i). At FIG. 5(a), an opening is defined in one ormore passivation layers 503 and 504 which exposes a surface of a chippad 502, typically made of aluminum. At least one intermediate layer 506and 507 is formed over the passivation layers 503 and 504 and over theexposed surface of the chip pad 502. The intermediate layers 506 and 507may be under bump metallurgy (UBM) layers. For example, the layer 506may be a UBM adhesion layer made of Cr, Ni or TiW, and the layer 507 maybe a wetting layer made of Cu, Ni or NiV. Also, an additional oxidationlayer of Au may be provided as well.

[0033] Next, at FIG. 5(b), a photoresist 515 is patterned over theintermediate layer 507 so as to expose one or more surface portions ofthe intermediate layer 507. In this embodiment, a plurality of openings516 in the photoresist 515 have a cross-section which defines a meshpattern in a plane parallel to the contact pad.

[0034] Then, at FIG. 5(c), a metal 511 is deposited, for example byelectroplating, so as to fill the openings to a given height. Examplesof the metal 511 include Ni, Cu, Pd, Pt and alloys thereof. Thephotoresist 515 is then removed, with the resultant structure having aplurality of metal projections 511 extending upward from the surface ofthe intermediate layer 507 as shown in FIG. 5(d).

[0035] Next, as shown in FIG. 5(e), another photoresist 517 is patternedwhich has an opening 518 that exposes the intermediate layer 507 and themetal projections 511. The opening 518 defines a solder bump region.Then, at FIG. 5(f), a solder material 505 is deposited so as to fill theopenings 518 to a given height. Examples of the solder material 505include Sn, Pb, Ni, Au, Ag, Cu, Bi and alloys thereof. The photoresist517 is then removed, with the resultant structure having the pluralityof metal projections 511 extending upwardly from the surface of theintermediate layer 507 and embedded within the solder material 505 asshown in FIG. 5(g).

[0036] Then, at FIG. 5(h), using the solder material 505 as a mask, anetching process is conducted for the purpose of removing theintermediate (UBM) layers 506 and 507 outside a region of the solderbump structure. This etching is carried out in the case where the UBMlayers extend continuously between adjacent solder bumps. Finally, atFIG. 5(i), the solder bump material 505 is heated at or above itsmelting point such that the solder bump material 505 is reflowed into aglobe-shape configuration.

[0037] A method for manufacturing a solder bump structure according toanother embodiment of the invention will now be described with referenceto FIGS. 6(a) through 6(g). At FIG. 6(a), an opening is defined in oneor more passivation layers 603 and 604 which exposes a surface of a chippad 602, typically made of aluminum. At least one intermediate layer 606and 607 is formed over the passivation layers 603 and 604 and over theexposed surface of the chip pad 602. The intermediate layers 606 and 607may be under bump metallurgy (UBM) layers. For example, the layer 606may be a UBM adhesion layer made of Cr, Ni or TiW, and the layer 607 maybe a wetting layer made of Cu, Ni or NiV. Also, an additional oxidationlayer of Au may be provided as well.

[0038] Next, at FIG. 6(b), a photoresist 615 is patterned over theintermediate layer 607 so as to expose one or more surface portions ofthe intermediate layer 607. In this embodiment, a plurality of openings616 in the photoresist 615 have a cross-section which defines a meshpattern in a plane parallel to the contact pad.

[0039] Then, at FIG. 6(c), a metal 611 is deposited, for example byelectroplating, so as to fill the openings to a given height. Examplesof the metal 611 include Ni, Cu, Pd, Pt and alloys thereof.

[0040] Next, at FIG. 6(d), a solder bump material 605 is deposited so asto fill the openings 616 in the photoresist. Examples of the soldermaterial 605 include Sn, Pb, Ni, Au, Ag, Cu, Bi and alloys thereof. Thephotoresist 617 is then removed, with the resultant structure having aplurality of stacked structures, each of the stacked structuresincluding a metal projection 611 and a portion of a the solder bumpmaterial 605 as shown in FIG. 6(e).

[0041] Then, at FIG. 6(f), using the solder bump material 605 and themetal projections 611 as a mask, an etching process is conducted for thepurpose of removing the intermediate (UBM) layers 606 and 607 fromoutside the solder bump region and from between the stacked structuresof the solder bump material 605 and the metal projections 611. Thisetching is carried out in the case where the UBM layers extendcontinuously between adjacent solder bumps. Finally, at FIG. 6(g), thesolder bump material 605 is heated at or above its melting point suchthat the solder bump material 605 is reflowed into a globe-shapeconfiguration.

[0042] Another embodiment of the present invention will now be describedwith reference to FIGS. 7(a), 7(b), 8(a) and 8(b). This embodiment ischaracterized by metal projections which are formed of dendritestructures instead of the columnar structures of the previousembodiments. In particular, after deposition of the intermediate (UBM)layers, a photoresist is patterned having an opening which exposes abump region of the intermediate layers. Then dendrite crystal structuresare grown within the bump region. In this regard, reference is made toU.S. Pat. No. 5,185,073, entitled “Method Of Fabricating NendriticMaterials”, which is incorporated herein by reference, and whichdescribes techniques which may be used to form a dendrite structurewithin the bump region according to the present invention. Examples ofmaterials of the dendrite structure of the present embodiment includeNi, Cu, Pd, Pt and alloys thereof.

[0043] After formation of the dendrite structure, the bump region isfilled with a solder material so as to embed the dendrite structurewithin the solder material. Then, the photoresist is removed, theintermediate (UBM) layers are etched outside the bump region, and thesolder material is reflowed to obtain the structure illustrated in FIG.7(a), and in FIG. 7(b) which a cross-sectional view along line II-II′ inFIG. 7(a). In these figures, the solder bump structure includes acontact pad 702 of an electronic device such as an IC chip 701. Anopening is defined in one or more passivation layers 703 and 704 whichexposes a surface of the chip pad 702. Interposed between a solder bump705 and the chip pad 702 are one or more intermediate layers 706 and707, such as UBM layers. The solder bump 705 is located over theintermediate layer 707 and includes a metal dendrite structure 711embedded therein. FIGS. 8(a) and 8(b) are photographs in a verticalcross-sectional plane of exemplary dendrite structures.

[0044] Any crack in the solder bump 705 will tend to propagatehorizontally through the bump material, and in this case, the metaldendrite projections 711 act as obstacles to crack propagation. Theseobstacles have the effect of increasing the crack resistance, andfurther lengthen the propagation path of any crack as it travels throughthe solder bump material, thus decreasing the likelihood device failure.

[0045] In the drawings and specification, there have been disclosedtypical preferred embodiments of this invention and, although specificterms are employed, they are used in a generic and descriptive senseonly and not for purposes of limitation, the scope of the presentinvention being set forth in the following claims.

What is claimed is:
 1. A method for manufacturing a solder bump,comprising: forming at least one metal protrusion extending upwardlyover a contact pad; and embedding the at least one metal protrusion in asolder material.
 2. The method as claimed in claim 1, wherein aplurality of metal protrusions are formed so as to extend upwardly overthe contact pad.
 3. The method as claimed in claim 2, wherein each ofthe metal protrusions is formed as a columnar structure having one of anelliptical or polygonal cross-section in a plane parallel to the contactpad.
 4. The method as claimed in claim 2, wherein each of the metalprotrusions is formed as a dendrite structure grown over the contactpad.
 5. The method as claimed in claim 1, further comprising, prior toforming the at least one metal protrusion, forming an under bumpmetallurgy (UBM) over the contact pad, wherein the UBM is comprised of ametal adhesion layer formed over the contact pad and a metal wettinglayer formed over the metal adhesion layer.
 6. The method as claimed inclaim 5, wherein the UBM is further comprised of a metal oxidationprotection layer formed over the metal wetting layer.
 7. The method asclaimed in claim 1, wherein the metal is selected from the groupconsisting of Ni, Cu, Pd, Pt or alloys thereof.
 8. The method as claimedin claim 1, wherein the solder material is selected from the groupconsisting of Sn, Pb, Ni, Au, Ag, Cu, Bi or alloys thereof.
 9. A methodfor manufacturing a solder bump, comprising: depositing an intermediatelayer over a contact pad; forming a photoresist over a surface of theintermediate layer; patterning the photoresist to define at least oneopening which partially exposes the surface of the intermediate layer;filling the at least one opening of the photoresist with a metal;removing the photoresist to expose the metal, wherein the metal forms atleast one metal protrusion extending upwardly from the surface of theintermediate layer; and embedding the at least one metal protrusion in asolder material formed over the intermediate layer.
 10. The method asclaimed in claim 9, wherein said embedding the at least one protrusioncomprises: forming a second photoresist which is patterned to expose asolder bump region over the intermediate layer and the at least onemetal protrusion; filling the solder bump region with the soldermaterial; removing the second photoresist; and reflowing the soldermaterial.
 11. The method as claimed in claim 9, wherein the photoresistdefines a plurality of openings, and wherein the metal forms acorresponding plurality of metal protrusions.
 12. The method as claimedin claim 11, wherein a cross-section of the plurality of openings in thephotoresist defines a mesh pattern in a plane parallel to the contactpad.
 13. The method as claimed in claim 9, wherein the intermediatelayer is under bump metallurgy (UBM) comprised of a metal adhesion layerformed over the contact pad and a metal wetting layer formed over themetal adhesion layer.
 14. The method as claimed in claim 13, wherein theUBM is further comprised of a metal oxidation protection layer formedover the metal wetting layer.
 15. The method as claimed in claim 9,wherein the metal is selected from the group consisting of Ni, Cu, Pd,Pt or alloys thereof.
 16. The method as claimed in claim 9, wherein thesolder material is selected from the group consisting of Sn, Pb, Ni, Au,Ag, Cu, Bi or alloys thereof.
 17. A method for manufacturing a solderbump, comprising: depositing an intermediate layer over a contact pad;forming a photoresist over a surface of the intermediate layer;patterning the photoresist to define at least one opening whichpartially exposes the surface of the intermediate layer; filling the atleast one opening of the photoresist with a metal to a first depth;filling the at least one opening of the photoresist with a soldermaterial to a second depth such that the solder material is stacked onthe metal within each of the at least one opening; removing thephotoresist to expose the metal and the first solder material, whereinthe metal and first solder material form at least one protrusionextending upwardly from the surface of the intermediate layer; andreflowing the solder material to form a solder bump having the metalembedded therein.
 18. The method as claimed in claim 17, wherein thephotoresist defines a plurality of openings, and wherein the metal andthe solder material form a corresponding plurality of protrusions whenthe photoresist is removed.
 19. The method as claimed in claim 18,wherein a cross-section of the plurality of openings in the photoresistdefine a mesh pattern in a plane parallel to the contact pad.
 20. Themethod as claimed in claim 17, wherein the intermediate layer is underbump metallurgy (UBM) comprised of a metal adhesion layer formed overthe contact pad and a metal wetting layer formed over the metal adhesionlayer.
 21. The method as claimed in claim 20, wherein the UBM is furthercomprised of a metal oxidation protection layer formed over the metalwetting layer.
 22. The method as claimed in claim 17, wherein the metalis selected from the group consisting of Ni, Cu, Pd, Pt or alloysthereof.
 23. The method as claimed in claim 17, wherein the soldermaterial is selected from the group consisting of Sn, Pb, Ni, Au, Ag,Cu, Bi or alloys thereof.
 24. A method for manufacturing a solder bump,comprising: depositing an intermediate layer over a contact pad; forminga photoresist over a surface of the intermediate layer; patterning thephotoresist to expose a solder bump region over the surface of theintermediate layer; growing at least one metal dendrite having aplurality of branches which project upwardly in the solder bump regionfrom the surface of the intermediate layer; filling the solder bumpregion with a solder material so as to embed the at least one metaldendrite within the solder material; and removing the photoresist. 25.The method as claimed in claim 24, further comprising reflowing thesolder material after removing the photoresist.
 26. The method asclaimed in claim 24, wherein the intermediate layer is under bumpmetallurgy (UBM) comprised of a metal adhesion layer formed over thecontact pad and a metal wetting layer formed over the metal adhesionlayer.
 27. The method as claimed in claim 26, wherein the UBM is furthercomprised of a metal oxidation protection layer formed over the metalwetting layer.
 28. The method as claimed in claim 24, wherein a materialof the metal dendrite is selected from the group consisting of Ni, Cu,Pd, Pt or alloys thereof.
 29. The method as claimed in claim 24, whereinthe solder material is selected from the group consisting of Sn, Pb, Ni,Au, Ag, Cu, Bi or alloys thereof.
 30. A solder bump structure,comprising: a contact pad; an intermediate layer located over thecontact pad; a solder bump located over the intermediate layer; and atleast one metal projection extending upwardly from a surface of theintermediate layer and embedded within the solder bump.
 31. The solderbump structure of claim 30, comprising a plurality of metal projectionsextending upwardly from the surface of the intermediate layer andembedded within the solder bump.
 32. The solder bump structure of claim31, wherein a cross-section of the plurality of projections define amesh pattern in a plane parallel to the contact pad.
 33. The solder bumpstructure of claim 31, wherein the plurality of metal projections aredendrites grown on the surface of the intermediate layer.
 34. The solderbump structure as claimed in claim 31, wherein a material of the metalis selected from the group consisting of Ni, Cu, Pd, Pt or alloysthereof.
 35. The solder bump structure as claimed in claim 31, wherein amaterial of the solder bump is selected from the group consisting of Sn,Pb, Ni, Au, Ag, Cu, Bi or alloys thereof.
 36. The solder bump structureas claimed in claim 31, wherein a melting point of a material of thesolder bump is less than a melting point of the metal.
 37. The solderbump structure as claimed in claim 31, wherein the contact pad islocated on a semiconductor chip contained a flip chip package or a waferlevel package.